1. Field of Invention
The present invention relates to a bidirectional level shifter, especially a bidirectional level shifter using a Schottky diode to provide a voltage difference.
2. Description of Related Art
A bidirectional level shifter is a commonly used signal input/output device, for example, used for bidirectional serial data transmission. FIG. 1 shows a bidirectional level shifter 10 disclosed by U.S. Pat. No. 7,822,890, wherein one of the signal terminal I/O (a) and the signal terminal I/O(b) can be a signal input terminal, and the other can be a signal output terminal. The bidirectional level shifter 10 includes two switches M1 and M2, an inverter 11, and a comparator 12. The voltage Vt is a low reference voltage between a logic high voltage level and a logic low voltage level of the input signal to the signal terminal I/O (b) when the signal terminal I/O(b) acts as the input terminal, and the voltage Vp is a low voltage slightly higher than the voltage Vt, for confirming the signal transmission direction to prevent any latch.
The bidirectional level shifter 10 operates as thus: when the signal terminal I/O (a) is the input terminal and at a low level, it turns on the switch M2 through the inverter 11 so that the voltage at the signal terminal I/O (b) is equal to the voltage Vp. As the signal terminal I/O(b) being the output terminal, the voltage Vp is judged to be a logic low voltage level, but because the voltage Vp is higher than the voltage Vt, the switch M1 is turned off and no latch problem will occur. When the signal terminal I/O (a) is the input terminal and at a high level, it turns off the switch M2 through the inverter 11, and the voltage at the signal terminal I/O (b) is raised to a high level by the voltage source Vddb; the switch M1 is also turned off and no latch problem will occur. When the signal terminal I/O (b) is the input terminal and at a low level, it turns on the switch M1 through the comparator 12 so that the signal terminal I/O(a) is at a low level close to the ground level. The low level at the signal terminal I/O (a) turns on the switch M2, but the signal terminal I/O (b) is the input terminal and is controlled by the input signal, so the output of the comparator 12 is still at high level. When the signal terminal I/O (b) is the input terminal and at a high level, it turns off the switch M1 through the comparator 12, and the voltage of the signal terminal I/O(a) is raised to a high level by the voltage source Vdda to turn off the switch M2.
The above prior art has the drawback that a regulator circuit is required to provide the voltage Vp in the bidirectional level shifter 10, but the regulator circuit occupies a large area in an IC chip and consumes current, and the NMOS switch in the regulator circuit requires a large area to reduce its body effect. Therefore, a simple and effective solution for reducing the circuit area and improving the operation efficiency is important and desired.